Radio packet communication uses Automatic Repeat reQuest for improvement in band use efficiency. In Automatic Repeat reQuest, if the result of decoding a received packet includes an error, the same data will be retransmitted. Since the same data is retransmitted, a receiver, which executes decoding with the use of Log Likelihood Ratio (LLR, which will be hereafter referred to as “likelihood information”) of a received packet, can improve a received signal in quality by combining the likelihood information of the initially transmitted packet with the likelihood information of a retransmitted packet.
On the other hand, a radio transmitter successively subjects transmitted data to a scrambling process and a pseudo-randomizing process, and then encodes the resultant data in many cases in order to make a signal smooth in spectrum and time waveform, or in order to make small interference to other systems. In the scrambling process, a portion of data in a single packet will be subjected to a replacement (or sign inversion) operation, in which “0's” are replaced with “1's” and “1's” are replaced with “0's”. The replacement of “0's” and “1's” is controlled by a scrambling code generated by a scrambling circuit, and the scrambling code is determined by a scrambling seed. The receiver has a descrambling circuit, which is the same in structure as the scrambling circuit of the transmitter, and subjects the decoded data to a descrambling process, thereby obtaining the original transmitted data. In some communication systems, however, a scrambling seed may change at the time of retransmission, and may be different from what it was at the time of initial transmission. Then, even if the retransmitted data is the same as the initial transmitted data, positions where “0's” and “1's” are replaced with the respective counter digits “1's” and “0's” will be different since the scrambling seed used at the time of retransmission is different from what it was at the time of initial transmission. As a result, the transmitted data and the retransmitted data will be received as a completely different data stream, so that the received data cannot be improved in quality even if likelihood information combination is executed.
With a view to solving the above problem, there is conventionally proposed a technique, in which a received signal is improved in quality with the use of likelihood information combination even if a scrambling seed has changed at the time of retransmission. When the initially transmitted packet is received in the conventionally proposed technique, a decoding process and a descrambling process are successively executed with the use of soft values (sign and amplitude), and a result (a soft value) obtained by the descrambling process is stored in a memory. When a retransmitted packet is received, the stored result of the descrambling process is subjected to a scrambling process with the use of the same scrambling seed as the retransmitted packet, and the scrambled result is coded. The scrambling process and the coding process are performed with the use of soft values. The coding result (the obtained soft value) is equivalent to the likelihood information of a packet which is transmitted after the initially transmitted data has been subjected to the scrambling process with the use of the same scrambling seed as the retransmitted packet. Therefore, if the coding result is combined with the likelihood information of the retransmitted packet, the reliability of the received signal will be improved.
The conventional receiver uses soft values when executing each of decoding, descrambling, scrambling, and coding, so that circuit structure and power consumption will be large. In a general radio communication device, an input to a decoder and an input/output of a descrambling circuit are binary data. Therefore, the conventional technique additionally requires an exclusive decoder and an exclusive descrambling circuit, each being exclusively used for processing soft values in order to achieve combination of likelihood information.